Vaccine hesitancy is a delay in acceptance or refusal of vaccines despite availability and supporting evidence of effectivity. The term also includes accepting vaccines but remaining uncertain about their use or using certain vaccines but not others. Although adverse effects associated with vaccines are occasionally observed, the scientific consensus that vaccines are generally safe and effective is overwhelming. Vaccine hesitancy often results in disease outbreaks and deaths from vaccine-preventable diseases. Therefore, the World Health Organization characterizes vaccine hesitancy as one of the top ten global health threats.